Implantable stimulation devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder subluxation, etc. FIG. 1 shows an implantable stimulation device 1 as may be used for spinal cord stimulation or deep brain stimulation. Such a device 1 typically includes an Implantable Pulse Generator (IPG) 10, which includes a biocompatible device case 12 formed of a conductive material such as titanium for example. The case 12 typically holds the circuitry and battery necessary for the IPG 10 to function, although IPGs can also be powered via external RF energy and without a battery. The IPG 10 is coupled to electrodes 16 via one or more electrode leads 18, such that the electrodes 16 form an electrode array 20. The electrodes 16 are carried on a flexible body which also houses the individual signal wires 24 coupled to each electrode. In the illustrated embodiment, there are eight electrodes on each lead 18, although the number of leads and electrodes is application specific and therefore can vary. The leads 18 couple to the IPG 10 using lead connectors 26, which are fixed in a non-conductive header material 28, which can comprise an epoxy for example.
FIG. 2 shows a first embodiment 201 of implantable stimulation device implanted in a patient for deep brain stimulation and a second embodiment 202 implanted in the patient for spinal cord stimulation. For deep brain stimulation, the IPG 10 is typically embedded in the in the patient's chest inferior to the clavicle. The signal wires 24 are routed beneath the skin of the patient's neck and head and the leads 18 are implanted into the patient's brain 32. For spinal cord stimulation, the IPG 10 is typically embedded in the in the patient's buttock and the leads 18 are implanted into the patient's spinal column.
The implantable stimulation device may further comprise a handheld Remote Control (RC) (not shown) to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters. The RC is used to send data to and receive data from the IPG 10. For example, the RC can send programming data to the IPG 10 to dictate the therapy the IPG 10 will provide to the patient. Also, the RC can act as a receiver of data from the IPG 10, such as various data reporting on the IPG's status. Wireless data transfer between the IPG 10 and the RC can take place via magnetic inductive coupling. To implement such functionality, both the IPG 10 and the RC typically have electrical coils that can act as the transmitter or the receiver, thus allowing for two-way communication between the two devices, as is well known in the art.
IPGs are routinely implanted in patients who are in need of Magnetic Resonance Imaging (MRI). Thus, when designing implantable neurostimulation systems, consideration must be given to the possibility that the patient in which neurostimulator is implanted may be subjected to electro-magnetic forces generated by MRI scanners, which may potentially cause damage to the neurostimulator as well as discomfort to the patient. In particular, in MRI, spatial encoding relies on successively applying magnetic field gradients. The magnetic field strength is a function of position and time with the application of gradient fields throughout the imaging process. Gradient fields typically switch gradient coils (or magnets) ON and OFF thousands of times in the acquisition of a single image in the present of a large static magnetic field. Present-day MM scanners can have maximum gradient strengths of 100 mT/m and much faster switching times (slew rates) of 150 mT/m/ms, which is comparable to stimulation therapy frequencies. Typical MRI scanners create gradient fields in the range of 100 Hz to 30 KHz, and radio frequency (RF) fields of 64 MHz for a 1.5 Tesla scanner and 128 MHz for a 3 Tesla scanner.
The strength of the gradient magnetic field may be high enough to induce voltages (5-10 Volts depending on the orientation of the lead inside the body with respect to the MRI scanner) on to the stimulation lead(s) 18, which in turn, are seen by the IPG electronics. If these induced voltages are higher than the voltage supply rails of the IPG electronics, there could exist paths within the IPG that could induce current through the electrodes on the stimulation lead(s), which in turn, could cause unwanted stimulation to the patient due to the similar frequency band, between the MM-generated gradient field and intended stimulation energy frequencies for therapy, as well as potentially damaging the electronics within the IPG. The gradient (magnetic) field may induce electrical energy within the wires of the stimulation lead(s), which may be conveyed into the circuitry of the IPG and then out to the electrodes of the stimulation leads.
Accordingly, the IPG may feature an MM-safe mode that protects the IPG 10 and the patient from damage or injury due to magnetic field-induced electrical energy when the patient undergoes an MRI. For example, in MRI-safe mode, the IPG may cease providing stimulation to the patient. Additionally, (or alternatively) the IPG may increase the voltage within the IPG to prevent unwanted induced current through the IPG. The IPG may modify or suspend other operations, such as passive charge recovery, an operation whereby charge is passively conveyed to AC ground by closing switches associated with the active electrodes. Within an MRI, the closed switches may potentially provide a path for magnetically induced currents into and out of the IPG. The recovery switches are therefore left open during MRI-safe mode.
The IPG can be set to MM-safe mode using the RC. Additionally, some IPGs can be set to MM mode by placing a magnet against the patient's skin over the IPG. Some IPGs include internal magnetic sensors, typically Hall effect magnetic sensors, that are capable of sensing an external magnetic field, such as an MM field. Hall effect magnetic sensor have a limitation in that they are unidirectional. In other words, a Hall effect magnetic sensor is only effective when it is situated in a particular orientation with respect to the magnetic field. Therefore, a series of three orthogonally oriented Hall effect sensors must be used to reliably sense the presence of an external magnetic field. Such a sensor design is undesirably large.